The ppp layer goes around calling the unthrottle method from non sleeping
paths. This isn't safe because the unthrottle methods in the tty layer need
to be able to sleep (consider a USB dongle).

Until now this didn't show up because the ppp layer never actually throttled
a port so the unthrottle was always a no-op. Currently it's a mutex taking
path so warnings are spewed if the unthrottle occurs via certain paths.